As such discharge tube, the applicant has previously submitted Japanese Utility Model Registration No. 3122357.
The discharge tube 60 as illustrated in FIGS. 10 and 11, includes a first electrode 62, a second electrode 64 and a third electrode 66 which are arranged in parallel, and an airtight outer enclosure 72 which includes a first cylindrical tube 68 made of an insulating material and sandwiched between the first electrode 62 and the second electrode 64 and a second cylindrical tube 70 made of an insulating material and sandwiched between the second electrode 64 and the third electrode 66.
In the second electrode 64 disposed in the center, a through-hole 74 is formed allowing the internal spaces of the first cylindrical tube 68 and the second cylindrical tube 70 to communicate with each other.
Moreover, the first electrode 62 and the third electrode 66 disposed on the left and right protrude toward a center of the airtight outer enclosure 72, respectively, and are provided with discharge electrode portions 76, 76 disposed inside the airtight outer enclosure 72. A discharge gap 78 is formed between the discharge electrode portions 76, 76 of the first electrode 62 and the third electrode 66. Hollow portions 62b and 66b are formed on the outer side surfaces of the first electrode 62 and the third electrode 66, respectively.
The first electrode 62, the second electrode 64 and the third electrode are airtightly joined with the first cylindrical tube 68 and the second cylindrical tube 70 through a sealant (not illustrated).
The circumferences 62a, 64a and 66a of the first electrode 62, the second electrode 64 and the third electrode 66, protrude to the outside from the outer peripheral surfaces 68a and 70a of the first cylindrical tube 68 and the second cylindrical tube 70, respectively.
A predetermined discharge gas is filled in the airtight outer enclosure 72.
In the discharge tube 60, when between the first electrode 62 and third electrode 66, a voltage equal to or higher than the discharge starting voltage of the discharge tube 60 is applied, a discharge is generated in the discharge gap 78.